Invalid IDREF attribute error

Hello!

I'm using Xercces-J 2.9.1.

I tried to validate the XML document that have two invalid IDREF type
attribute 'xref' elements.

 <para>This is a test.<xref linkend="aaa"/><xref linkend="bbb"/></para>

But Xeces reports only one error.

 'An element with the identifier "bbb" must appear in the document.'

Why Xerces does not report all of the unresolved IDREFs? Is this the
limitation?

Any suggestions are welcome.

Thanks,

Toshihiko Makita